Liquid crystals (LCs) are anisotropic elastic materials that are comprised of molecules with translational mobility similar to that of isotropic fluids and long-range orientational ordering that is reminiscent of solid crystals[1]. Synthetic liquid crystals are a particularly promising but largely unexplored class of materials for engineering dynamic and responsive interfaces between synthetic and biological systems because interfacial events involving biomolecular and/or mechanical interactions can lead to reorganization of the LCs.[2-4] Such reorganization of the LCs can be transduced optically, as was demonstrated when human embryonic stem cells (hESCs) were cultured on the protein-coated surfaces of thermotropic LCs.[4] Whereas hESCs are known to grow on very soft materials (e.g. gels with shear modulus, G, <35 Pa),[5] many other types of mammalian cells require a level of mechanical rigidity in their underlying substrates that exceeds that of low molecular weight LCs in order to maintain normal cell functions.[5, 6]
Synthetic LCs have received attention as materials with which to create responsive interfaces to biological systems.[2-4, 12-15] Examples include the use of LCs to report the presence of viruses captured at surfaces,[12] amplify receptor-ligand binding events involving proteins,[2] and report enzymatic activity.[3, 13] Additionally, LCs have been studied as materials to create interfaces to mammalian cells.[4, 15] Fang et al. demonstrated that the thermotropic liquid crystal 4-cyano-4′-pentylbiphenyl, 5CB, assumed distinct orientations when it was layered over several different cell lines cultured on glass substrates.[14] However, those cells were fixed (i.e. dead) prior to contact with the LCs. In a later study by Luk et al.,[15] CB was shown to cause cell death within a short time period when placed into direct contact with live cells. To address the issue of cytotoxicity of LCs, Luk et al. screened a panel of LCs[15] and identified the nematic LC called TL205 that exhibited no cytotoxicity when fibroblasts were incubated under it for up to 4 hr. TL205 is a mixture of mesogens containing cyclohexane-fluorinated biphenyls and fluorinated terphenyls with aliphatic chains containing 2-5 carbon atoms (E Merck Co, Germany). Building from the study by Luk et al., Lockwood et al. demonstrated that ˜20 μm thick films of TL205, when hosted within electron microscopy grids and coated with molecularly thin (thickness of ˜10 nm) films of an extracellular matrix protein mixture (Matrigel), could support the growth and self-renewal of hESCs.[4] Reorganization of the Matrigel over time by the hESCs was found to trigger an orientational transition within the film of LC that could be readily seen by using polarized light transmitted through the LC.
Terentjev, Cates and others have demonstrated that suspensions of sterically stabilized polymethylmethacrylate (PMMA) particles (diam=200-1000 nm) dispersed in isotropic 5CB, i.e. above the nematic-isotropic phase transition temperature TNI (T>TNI, with TNI˜35.5° C. for 5CB), when cooled to room temperature, form a network within the nematic 5CB, thereby creating a birefringent waxy composite with storage moduli of G′˜1-10 kPa (30° C. with 5-15 wt % of colloids).[9-11]